In recent years, the spread of contactless communication systems using contactless communication techniques such as NFC (Near Field Communication) and FeliCa (FeliCa is a registered trademark in Japan, other countries, or both) has been remarkable. In a contactless communication system such as mentioned above, communication (i.e., transfer of data or electrical power) is performed through magnetic field coupling of a coil antenna incorporated into a transmission device (R/W (Reader/Writer) device) and a coil antenna incorporated into a reception device (card-type/tag-type).
In a generally known technique for enabling a longer communication distance between a transmission device and a reception device through use of electromagnetic induction, antennas of the transmission device and the reception device are each a resonant antenna (resonant circuit) in which a resonant capacitor is connected to a coil antenna. This technique is also being used for wireless power supply, the specifications of which are drawn up by standardizing bodies such as the WPC (Wireless Power Consortium) and the PMA (Power Matters Alliance).
The magnitude of magnetic coupling (coupling coefficient k) between coil antennas of a transmission device and a reception device changes depending on the distance between the antennas and the size of the antennas in these devices. As explained above, communication between the transmission device and the reception device is through magnetic coupling of the coil antennas in these devices and, as a consequence, a change in the coupling coefficient k leads to a change in communication characteristics.
PTL 1 discloses a technique in which an R/W device (transmission device) monitors information pertaining to current flowing in a coil antenna of a transmission section while transmitting a signal from the transmission section, determines a transmission state based on the monitored information, and adjusts communication characteristics of the transmission section based on the communication state that is determined.
PTL 1 discloses that when the current flowing in the coil antenna in a situation in which the R/W device is used by itself is taken to be a value of 1, placing a contactless IC card (reception device) in opposition to the R/W device leads to a decrease in the current flowing in the coil antenna of the R/W device. Furthermore, PTL 1 discloses that there is a correlation between the percentage decrease in the current flowing in the coil antenna and the distance between the R/W device and the contactless IC card. PTL 1 also discloses that the voltage induced in the contactless IC card (induced voltage) increases in accompaniment to a decrease in the current flowing in the coil antenna of the R/W device when a relative value of the current flowing in the coil antenna of the R/W device is from 1 to 0.5, that the induced voltage is at a maximum when the relative value is 0.5, and that the induced voltage decreases when the relative value becomes smaller than 0.5.
The decrease in the current flowing in the coil antenna of the R/W device is not dependent on the antenna size or the Q factor of a resonant circuit. In the technique disclosed in PTL 1, the communication state (i.e., whether the communication distance is a long distance, a medium distance, a short distance, or close contact) is determined based on the information pertaining to the current flowing in the coil antenna. Furthermore, in the technique disclosed in PTL 1, the communication characteristics of the transmission section (for example, the resonant frequency and the Q factor of the resonant circuit) are adjusted based on the communication state that is determined in order to optimize the communication characteristics of the transmission section in accordance with the communication state and enable stable communication.
PTL 2 discloses a technique in which a characteristic of a resonant circuit (capacitance of the resonant circuit) that is included in a contactless IC card (reception device) in order to perform contactless communication with an R/W device (transmission device) is changed in accordance with an induced voltage in the contactless IC card.
PTL 3 discloses a technique in which the resonant frequency of a resonant circuit section that is included in an information processing terminal (reception device) in order to perform contactless communication with a reading and writing device (transmission device) is changed in accordance with a reference voltage and an induced voltage in the information processing terminal.
The techniques disclosed in PTL 2 and PTL 3 enable improvement of communication characteristics through changing of a characteristic of a resonant circuit in a reception device in accordance with an induced voltage in the reception device.
PTL 4 discloses a technique in which, in an information processing device (transmission device) including a communication antenna for performing contactless communication and in which either or both of a Q factor and a tuning frequency are variable, the distance between the information processing device and a communication device (reception device), such as an IC card, is estimated by detecting a voltage applied to the communication antenna and comparing the detected voltage to a threshold value, and the Q factor and/or the tuning frequency of the communication antenna is set in accordance with a result of the estimation.
When a transmission device and a reception device become too close, this generally causes tight coupling of an antenna of the transmission device and an antenna of the reception device, which may decrease a modulation factor and lead to communication failure. In the technique disclosed in PTL 4, when the voltage applied to the communication antenna falls below the threshold value, it is determined that the information processing device and the communication device have become too close, and the Q factor and/or the tuning frequency of the communication antenna is controlled such as to inhibit communication failure.